A number of attempts to improve the linear density (bits per millimeter, or bpmm) of binary data that can be reliably written on magnetic media, for example disks, exist in the art. One of these techniques is known as partial response coding or signaling.
The publication IEEE Transactions on Communications, Vol. Com-23, No. 9, September 1975, describes a number of partial response systems, which systems provide either 3 signal levels or 5 signal levels. This publication is incorporated herein by reference for its background teaching of partial response signaling.
The publication IBM Journal of Research and Development, July 1970, at pages 368-375, describes the "Application of Partial-response Channel Coding to Magnetic Recording Systems". This publication is also incorporated herein by reference.
Partial response signaling (sometimes called correlative level coding) is a form of pulse amplitude modulation that is used to convey digital information, wherein the effect of a known amount of intersymbol interference can be eliminated, because the interference is a known quantity.
Class IV partial response coding (PRIV) is a particular 3 signal level class of this coding technique (i.e. equivalent signal levels +1, 0 and -1). PRIV is a technique with which the present invention finds utility. As those skilled in the art will appreciate, the present invention can be readily extended to other techniques, such as the 5 signal level technique above mentioned, as by the use of 4 threshold circuits, rather than the use of two threshold circuits as disclosed herein for use with a 3 signal level PRIV technique. Stated differently, a partial response coding technique having N+1 critical signal levels requires N threshold circuit paths of the invention.
U.S. Pat. No. 4,504,872 is incorporated herein by reference with respect to its discussion of Class IV partial response signalling technique.
Whatever the prior partial response technique that was used to code and then magnetically record the binary data, subsequent reading of the magnetic media, and detection of the resulting analog read signal, requires the use of a clocking means in order to place the binary 1's and 0's in their proper time positions. This enables the original binary data to be reconstructed or recovered from the analog read signal.
A well known prior method of detecting magnetically recorded binary data is to detect the time-position of analog read signal peak magnitudes (i.e. recorded transitions), and then relate the time-positions of these peak magnitudes to a standard clock that is continuously synchronized with the binary data being recovered. This clock establishes sequential data sample periods or windows into which the read signal's continuously changing amplitude is mapped as binary 1's and 0's.
In these well known detecting schemes, the binary data has been coded (for example, NRZI coding) such that the recorded binary pattern has sufficient signal transitions to maintain the correct clock frequency. The clocking function for such a prior system is usually provided by a variable frequency oscillator (VFO), or a voltage controlled oscillator (VCO).
In these prior self clocking systems, when the time-position of the peaks in the analog read signal are determined to be out of phase with the clock transitions, a current is produced whose magnitude is proportional to this phase error. The phase error current is then averaged, filtered, or integrated, and the result is used to adjust the clock in a manner to reduce the phase error to zero.
Advanced coding techniques, of which Class IV partial response is an example, intentionally provide controlled intersymbol interference. This interference provides a read signal whose peak amplitudes are sometimes properly time-positioned, and at other times are positioned between data sample times, depending upon the particular data stream being read or received. While these advanced techniques have the advantage of providing high data packing rates, they complicate the generation of a data clocking signal.
For example, detection of partial response coded binary data results in critical analog read signal amplitudes and/or peaks sometimes occurring at clock-determined sample points, and sometimes occurring between these sample points. This variation in timing is due to the large partial response intersymbol interference, which interference, in accordance with these advanced coding techniques, is used in a constructive manner for coding efficiency. However, this same timing variation makes the use of conventional VFO/VCO detection techniques difficult, at best.
Some prior art partial response detection schemes use a separate, highly equalized signal channel, in an attempt to move the signal peaks back toward the time-position in which they were originally recorded. This arrangement generates electronic noise, tending to make this separate channel's signal useless for data detection, but usually adequate for controlling the VFO/VCO. In these schemes, the phase between the separate signal channel for controlling the VFO/VCO, and another separate signal channel that is used for data detection, must be compensated. As a result of a change in circuit components with age, the system tends to degrade with time, due to phase error. Another problem that can occur with this scheme is that the noise that is generated in the VFO/VCO signal channel may cause the clock's data detection window to shift in the wrong direction.
An example of prior art partial response detection schemes can be found in the publication IEEE Transactions on Magnetics, September 1984, VOL. MAG-20, Number 5, at pages 698--702.
A prior art class IV detection scheme whose clock recovery is based upon threshold crossing detection of the ternary waveform is described in an article entitled "High Data Rate Magnetic Recording in a Single Channel", found in the publication Fifth International Conference on Video and Data Recording, Apr. 2--5, 1984, at pages 151-157. While this publication describes threshold detection at two values identified as plus 1 and minus 1, these two values being one half of the nominal signal level, no suggestion is made in this publication relative the use of phase detectors, which are responsive to the outputs of the threshold detectors and to the output of a clock, the outputs of the phase detectors in turn controlling the clock, as is taught by the present invention.